What is "visual binary"

n. (context star English) A binary star whose components can be visually resolved.

A visual binary is a gravitationally bound system that can be resolved into two stars with the aid of a telescope. These stars are estimated, via Kepler's 3rd law, to have periods ranging from a number of years to thousands of years. A visual binary consists of two stars, usually of a different brightness. Because of this, the brighter star is called the primary and the fainter one is called the companion. If the primary is too bright, relative to the companion, this can cause a glare making it difficult to resolve the two components. However, it is possible to resolve the system if observations of the brighter star show it to wobble about a centre of mass. In general, a visual binary is resolved into two stars with a telescope if their centres are separated by a value greater than or equal to one arcsecond.

For a visual binary system, measurements taken need to specify, in arc-seconds, the apparent angular separation on the sky and the position angle- which is the angle measured eastward from North in degrees- of the companion star relative to the primary star. Taken over a period of time, the apparent relative orbit of the visual binary system will appear on the celestial sphere. The study of visual binaries reveal useful stellar characteristics: Masses, densities, surface temperatures, luminosity, and rotation rates.